Acetaminophen (APAP) overdose is the most common cause of acute liver failure (ALF) in the United States today. The toxicity is mediated by metabolism of the parent drug to the reactive species, N-acetyl-p-benzoquinone imine (NAPQI). At therapeutic doses NAPQI is detoxified by glutathione (GSH); however, following toxic doses, GSH is depleted and NAPQI covalently binds to hepatic proteins. The binding of APAP to protein forms APAP protein adducts, which are well-recognized biomarkers of toxicity in experimental models of APAP toxicity, in man, the diagnosis of APAP toxicity is typically based on the measurement of APAP in plasma samples of patients who present in the immediate time period after overdose. However, in patients who present late for medical management or in patients with ALF and encephalopathy, plasma APAP levels may be undetectable; the diagnosis is dependent therefore on historical and clinical data alone. Similarly, in the setting of "therapeutic misadventures," plasma APAP may not be detectable and the diagnosis is dependent upon dosing histories provided by the patient or family. We recently developed a very sensitive assay for the measurement of APAP protein adducts in human blood samples. The assay correctly identified APAP toxicity cases and negative controls and detected the presence of adducts in patients with late presentations to medical centers. In addition, the assay detected adducts in approximately 20% of patients with ALF of unknown etiology. The following application will allow for the transfer of the current research laboratory assay into a clinically useful, rapid turn around test. Analysis of samples from an existing sera bank of ALF patients will be performed to characterize these adducts in multiple patient populations. We will also establish a national laboratory for the assay and monitor its clinical applications. The data generated from this study will lead to enhanced recognition of APAP hepatotoxicity, thus allowing for better identification of "at risk" populations and the creation of prevention strategies.